1. Field of the Invention
The present invention relates to a form measuring apparatus and a method of registering a rotary table coordinate system.
2. Description of Related Art
A coordinate measuring machine system that includes a rotary table is known. The following advantages, for example, are imparted by including the rotary table. First, measurement efficiency is improved because a plurality of surfaces of a measured object can be measured by a single stylus tip. Second, a measurement region can be enlarged. For example, when a rotary table is provided to a horizontal-type coordinate measuring machine (used as the coordinate measuring machine), the measurement region is approximately doubled. Third, simultaneous scanning measurement is enabled, performed by the three axes of the coordinate measuring machine and an additional fourth axis of a rotary table axis. For example, Japanese Patent Laid-open Publication No. H03-84408 discloses a method of maintaining a constant angular relationship between a work piece and a probe during scanning measurement in simultaneous scanning measurement in four axes.
In a case where the rotary table is used to perform measurement, a coordinate system for the rotary table must be registered before beginning. A master ball (standard ball) fixated to one end on a top surface of the rotary table is typically used to register the rotary table coordinate system. For example, the rotary table is rotated 0°, 120°, and 240° from a predetermined angle, and a center position of the master ball is measured at each rotation angle. The center of a circle passing through the three measured center positions and a normal line of a circular plane passing through the center of the circle are calculated. The center of the circle is registered as an origin point of the rotary table coordinate system and the normal line is registered as a Z axis of the rotary table coordinate system.
However, in a case where there is run-out in the rotation axis of the rotary table, when the above-described method of registering the rotary table coordinate system is used, when the diameter of a round cylindrical shaft is measured at a position far from the top surface of the rotary table and a position near the top surface of the rotary table, the shaft being arranged such that an axis direction thereof is perpendicular to the top surface of the rotary table, a measured value in the far position becomes larger than the measured value in the near position. Specifically, the above-described method of registering the rotary table coordinate system does not accommodate run-out of the rotation axis, and errors become significant.
Japanese Patent Laid-open Publication No. 2009-271030 discloses a method of registering a rotary table coordinate system capable of inhibiting errors due to run-out of a rotation axis. In this method, a Z axis of the rotary table coordinate system is calculated using two master balls at different distances from a top surface of a rotary table. Specifically, the rotary table is rotated 0°, 120°, and 240° from a predetermined angle, and a center position of a first master ball and a center position of a second master ball are measured at each rotation angle. The center of a first circle passing through the three measured center positions of the first master ball, the center of a second circle passing through the three measured center positions of the second master ball, and a straight line passing through the centers of the first and second circle are calculated. The center of the first circle is registered as an origin point of the rotary table coordinate system and the straight line is registered as the Z axis of the rotary table coordinate system.
As noted above, according to the method of registering the rotary table coordinate system according to Japanese Patent Laid-open Publication No. 2009-271030, by registering the straight line that passes through the centers of the first and second circles as the Z axis of the rotary table coordinate system, errors due to run-out of the rotation axis can be inhibited. However, the inventors of the present invention have discovered that in both of the above-described methods of registering rotary table coordinate systems, a degree of accuracy for a registered value of the origin point in the rotary table coordinate system can be improved.